Radar is used to detect and locate remote objects by the reflection of radio waves. This is accomplished by transmitting a signal of a known waveform and observing the nature of the received echo. Traditionally, the radar waveform has usually been a repetitive train of short-duration pulses all of a constant amplitude. There are certain applications for radar, however, where the production of a pulse train or waveform having varying degrees of amplitudes is advantageous. For example, tracking of objects can be improved through the use of a radar system which is capable of producing variable waveforms. This is because such systems are less susceptible to jamming and other types of Electronic Counter Measures (ECMs).
The desirability of varying a waveform's shape, also known as pulse shaping, is discussed in Chapter 27 of RADAR TECHNOLOGY by Brookner entitled "EFFICIENT SPECTRUM CONTROL FOR PULSED RADAR TRANSMITTERS". Here, waveform control or spectrum control is discussed in the context of meeting Electro-Magnetic Compatibility (EMC) requirements which are now commonly specified on radar equipment, e.g., MIL-STD-469. Techniques are considered to improve the efficiency on shaped pulses for the Traveling Wave Tube (TWT) and Klystron Tube Transmitters. With these techniques, a reasonable efficiency can be achieved, however, the complexity in the design of the power supplies for the tubes is greatly increased. This translates into a significant cost increase which has prohibited widespread implementation.
Without the help of complex and expensive power supplies, tube transmitters have significant problems when they are run in any state other than saturation. As such, they are seldom operated except in that manner. Thus, with only this single mode of operation, tube transmitters have little capability to vary the power transmissions of the pulses. One way of varying the power is to form a series or sequence of transmitter tubes called a transmitter chain. Again, such a method is prohibitive because of its cost.
The amplitudes of pulses from tube transmitters may also be controlled at IF and microwave frequencies by using a PIN diode as an attenuator or switch. For example, a PIN diode can be configured in shunt to an output RF signal so that when the diode is zero biased the signal will be transmitted with no effect; when the diode is forward biased the RF resistance decreases, shunting some of the signal to ground and thus attenuating the output signal. This is the basic reflective attenuator. The disadvantage of such a circuit is that some of the input signal will be reflected back to the input port, resulting in reduced efficiency and a poor Voltage Standing Wave Ratio (VSWR) that increases as the current is increased. In addition, the attenuation range is limited. Other methods of amplitude control for tube-type transmitters are described in MODERN RADAR TECHNIQUES edited by Scanlan, Macmillan Publishing Co. 1987, at pages 63-69, however, each has limitations similar to those methods previously described.
Because of the above-mentioned drawbacks, there has been little interest in modulating the power in a transmit pulse from a tube transmitter, either as a function of pulses in a group or on an intrapulse basis, i.e., where various amplitudes are present in a single pulse envelope. Consequently, waveform designers have concentrated on the design of coded pulses of constant amplitude and placing these constant amplitude pulses in groups which are weighted upon their receipt at the receiver. This technique can be used to control the time or spectrum sidelobes of the transmitted wave.
Because tube transmitters are seldom operated except in saturation, it has also been impossible to match the transmitted pulse waveform to that of a rectangular pulse which would provide for optimum detection; that is because a rectangular transmitted pulse requires an unrealizable rectangular filter. This unmatched condition leads to losses in the system design, since essentially some of the transmitted power is being discarded. Also, because the tube transmitters have very fast risetimes, they are vulnerable to electronic countermeasures (ECM) or jamming.
Another type of transmitter which is gaining popularity and which is replacing tube transmitters in certain applications is the solid-state transmitter. In a tube transmitter, a single thermionic device or high power microwave valve provides the final amplification for the system. Having a single power device to provide system-wide power can be disadvantageous because such devices have a limited life and failure results in complete radar system failure. With solid-state transmitters, the available architectures are significantly more diverse. They range from a set of modules which are centrally located and which emulates a tube transmitter, to a fully distributed architecture which places transmit/receive modules in an array on the antenna. Such architectures allow for the use of smaller, less stressed and less costly mass produced microwave semiconductor power devices. The very long operating life of semiconductor devices not only allows a decrease in overall radar failure rate, but provides a gradual degradation in radar performance as the semiconductor devices fail.
With the advent of solid-state transmitters, system designers have new opportunities. A typical new opportunity is more flexibility in the transmitted waveform. Additional flexibility in waveform design is available, because with modular solid-state transmitters, the number of modules which are active at a given time is selectable. This selectability is useful in order to vary the power and amplitude of the transmitted pulses.
It is, therefore, an object of the present invention to provide an apparatus and method for utilizing the added flexibility of solid-state radar transmitters to produce advanced waveforms having varying degrees of amplitude, while at the same time reducing prime power usage. The apparatus achieves significant performance advantages, such as a reduction in prime power usage. As mentioned, radar systems capable of transmitting variable waveforms provide better detection because they are less susceptible to interference from jamming and other ECMS. This invention overcomes the difficulties associated with tube transmitters through the use of a variable combiner able to achieve modulation on a pulse-to-pulse basis or an intrapulse basis. The combiner increases efficiency by controlling losses in output power when individual modules of a solid-state transmitter are activated and deactivated. Intrapulse modulation and pulse to pulse modulation can be produced with the disclosed modular solid-state transmitter because the number of modules and the sequence in which they are activated is selectable. These selections provide the various desired levels of transntitted power and thus effectively achieve modulation.
In order to achieve this intrapulse modulation, a variable combiner is introduced whose coupling factor is coordinated with the module activations. In one embodiment, variable combining is accomplished by the use of RF switching logic for combining pairs of signals in parallel. In this case, RF switches are programmed to act as a conventional combiner when all modules are active. When a module is deactivated, the switches are set so that the active modules are connected to the output or the next stage of combining. Another embodiment uses a series approach to accomplish such variable combining. Here, when all modules are active RF switches and phase shifters are programmed for performance as in a conventional combiner. When a module is deactivated, the phase of the signals within each coupler section is adjusted by means of RF amplifier programming and phase shifters to avoid loss in the termination resistors.
Finally, since the solid-state transmitter is modular, different modules can be activated during the beginning and end portions of a waveform to accomplish the intrapulse modulation. This decreases the stress on any one individual module, achieving the same result as if identical modules were activated at both the start and finish of the modulated pulse.